1. Field of the Invention
The invention relates to the field of turbomachine rotor wheels, and in particular compressor wheels. In turbomachines, rotor wheels associated with stator wheels form compressor stages having the function of compressing the fluid passing therethrough. The design and optimization of a cascade of rotor wheels (i.e. a sequence of one or more rotor wheels), e.g. for a compressor, requires two objectives to be taken into account.
2. Description of the Related Art
The first objective is to have good compression efficiency. Compression efficiency may be defined as the ratio of the energy that would ideally be imparted to the fluid by isentropic compression from upstream to downstream of the cascade of rotor wheels divided by the energy actually imparted to the fluid. (In the present document, upstream and downstream are defined relative to the normal flow direction of the fluid through the cascade of rotor wheels.)
The second objective is to guarantee sufficient “surge margin”. Surge is a phenomenon of fluid instability that occurs within a compressor, giving rise to low frequency oscillations in the flow, and occurring when the flow rate, supply, pressure, and temperature conditions depart from the normal operating range of the turbomachine. Since this instability phenomenon usually develops a large amount of energy, it subjects the turbomachine to high levels of stress (statically and dynamically). It can thus readily be understood that a constant objective during the development of a cascade of rotor wheels is to extend its normal operating range as much as possible so that the compressor or the turbomachine of which it forms a part has a “surge margin” that is sufficient and that enables surge phenomena to be avoided.
In known manner, specific arrangements are made for rotor wheels in order to optimize the second objective, specifically optimizing surge margin.
In a rotor wheel or bladed rotor wheel, the radial clearance in operation between the stationary casing and the moving blades gives rise to a secondary flow referred to as the clearance flow. This flow gives rise to significant losses in the efficiency of rotor wheels and, in a majority of cases, may give rise to the loss of stability in the compressor (the surge phenomenon). Thus, in order to satisfy the second above-mentioned objective and maximize the surge margin of the rotor wheel cascade, it is known to treat the inside wall of the casing where it faces the ends of the rotor wheel blades.
By way of example, casing treatment may consist in forming a set of grooves in the inside wall of the casing. By means of these grooves, the surge margin of the rotor wheel is improved. Patent GB 2 408 546 thus provides an example of turbomachine casing treatment. Nevertheless, in the casing treatment disclosed therein, the arrangement of the grooves is very particular: the grooves are not circumferential, but are slots that are spaced apart circumferentially from one another with an angle of inclination relative to the radial direction that varies. As a result, the casing is relatively complex to fabricate and is therefore expensive, but without that ensuring that the casing can simultaneously increase surge margin and optimize compression efficiency.
In practice, most casing treatments are intended solely for optimizing the surge margin of the compressor without worrying about the often negative impact that has on compression efficiency.